Indirect water heater with electric element? Have oil and coal boilers

Newbie here, but have read lots of posts. Need your advice on designing the best setup for heating water at my folks' house.

Current hot water and heating in the house (northeastern Pennsylvania) is an oil burner and a coal burner. The coal burner operates about 6 months a year to provide heat to the house by pumping water up through the radiators. The oil burner can provide steam heat to the house, but it is used for that purpose only when they leave on periodic vacations throughout the winter. The rest of the year, it provides hot water to the house through it's tankless system. Not very efficient, especially with oil prices now at ~$3.70/gallon, it basically costs about $2,000+/year for hot water.

Total heating costs for the winter are ~$400 for coal. That's it. And even with that, they sometimes have doors open.

They want to keep the oil burner to prevent the house from freezing in the winter while they are away periodically and need to let the coal burner go out.

So the water heater alternatives as I see them are to:

1) install an electric tank only for full-year use
2) install an electric and an indirect in series or parallel, getting heat from the coal burner, each getting used 6 months/year
3) install an indirect with a built-in electric element (if they make such a thing)

Am I missing any other options? I've searched online, but haven't been able to find any indirect heaters with an option for a built-in electric element. Are they out there?

There is no gas service, and I am doubtful the up-front costs of an electric on-demand heater is worthwhile.

Avoid the tankless. They cost a mega ton to buy and install, the require frequent repairs and repairmen are hard to find, and the operating costs are huge. I think the best course of action would be the combination coal furnace for the winter and an electric heater for summer if the transition between them could be handled OK. Since the coal fired furnace is being used for heating the house, it costs nothing extra to heat the water in the winter. The pros may well have some additional (better) ideas.

If you installed two tanks, you'd probably want the electric in series after the indirect. That way, when the boiler was on, the electric would be fed with hot water, and may never have to turn on..and, if it did, would only have to compensate for standby losses, not heating the full tank from cold (which it would have to do when the boiler was down). The indirect would also act like a buffer and warm the water up some even though the boiler may not be on if it sat in there inbetween uses. You don't want them in parallel where you turn one off for 6-months, the water could get pretty rank.

I've not looked for an indirect with backup heating...I know of some that have dual input coils, but those are designed for solar with backup boiler; not electric.

Thanks for the suggestions. I agree that heating with coal during the winter seems like a no-brainer. Good point about an empty heater going rank after 6 months of non-use. Hadn't considered that.

At the very least, they do want to run the oil burner periodically, briefly, during the non-winter months just so that it remains in operating condition for when they need it in the winter. It's probably 40-50 years old. So another option we just thought of was to circulate water from the tankless heater in the oil burner to heat the indirect during the summer. However, my Dad pointed out that we would need to somehow put a control on the oil burner to get hot enough to heat the indirect, but not hot enough to start producing steam (particularly if there was a large demand at some point). Producing heat would end up heating the house, which is not good in the middle of the summer when it's already blistering hot and humid.

Has anyone ever done this? It is my understanding that this would be a much more efficient way to heat water using the oil burner vs. the existing built-in tankless, correct?

I have an oil-fired boiler with a tankless coil. It was using so much fuel in the summer just to keep the boiler hot that it cost more that it would cost for electricity.

I installed a standard electric water heater in series with the tankless coil so the tankless coil feeds the cold-water inlet of the water heater. The water from the boiler is usually hotter than the setting of the electric water heater so the electric unit doesn't do anything during the heating season.

I shut off the boiler at the end of the heating season and the electric takes over the job.

I installed a by-pass circuit so I can operate either with the other isolated for maintenance but have not done anything else. In two years the only operating requirement has been to turn off the boiler switch at the end of the season and turn it on when I need heat in the fall.

The setup also solved a problem that occurred occasionally in the winter when feed water is 33 F, and the tankless coil had trouble keeping up with peak shower flows. The average flow was well within the capacity of the tankless and by the time the water gets mixed in the 40 gallon water heater it is all just fine.

I have a small bronze circulator to add temperature to the electric tank unit by circulating from the tankless coil but have not needed it and have not installed it.

The electric water heater costs a lot less than an indirect heater and there is no need for both an indirect and an electric water heater.

Electricity in the summer costs less than the oil I was using in the summer to keep the boiler hot.

If your coal-fired boiler doesn't have a tankless coil (From the original post I suspect it doesn't) you could accomplish the same thing by installing an external heat exchanger to supply the electric water heater in the manner described above. The only indirect water heater that I found with an electric element was a very large and expensive Bryan brand.

I took a look at the Bryan heaters, and they look like all industrial models. Bummer.

I only think I'd consider oil to heat water if it could be accomplished by utilzing the oil boiler, since it has to be run sporatically anyway to keep it in operating condition. In comparing an oil water heater like the one in that link vs. a conventional electric, my guess is that the electric one will be cheaper to operate.

The coal boiler does not have a tankless coil installed. It's a small cast iron boiler (probably 70-90 years old) with a water jacket around it. It's a round boiler, maybe 2 feet in diameter by 4 feet tall.

Bob - Thanks for your detailed response. You said there is no need for an electric and an indirect heater. The only reason we're considering installing both is because the coal would heat the indirect for 6 mos out of the year since it's running anyway to heat the house. So that's basically 6 mos of free water water. For the 6 mos during the summer, we either have to heat with electric (a separate tank, or ideally, an electric element in the indirect), or somehow circulate water from the oil burner to heat the indirect.